Sliding furnace boat apparatus

ABSTRACT

An improved furnace boat has been developed for the epitaxial solution growth of III-V compounds. The boat comprises one or more closed compartments wherein the growth solution or solutions are located, in combination with a sliding substrate support member at the base of the solution compartments, such that the substrate wafer can be moved quickly and easily into contact with, or out of contact with, the growth solution. The technique permits optimum surface protection of the substrate prior to immersion, removal of the slice at any time increment of the growth and/or cooling cycle, and will ensure complete removal of gallium solution from the grown surface without damage to the surface.

United States Patent 1 Stone et al.

SLIDING FURNACE BOAT APPARATUS Inventors: Louis Earl Stone, Richardson;

Roberto Romano, Dallas, both of Tex.

Assignee: Texas Instruments Incorporated,

Dallas, Tex.

Filed: May 28, 1971 Appl. No.: 147,998

US. Cl 118/415, 118/412, 118/422, 118/109 Int. Cl. B05c 3/09, H011 7/38,H011 7/46 Field of Search 118/412, 415, 500, 118/109, 422; 15/268;101/35, 41; 148/171, 172; 23/273 Nelson 148/171 X [111 3,747,562 [4 1July 24, 1973 Primary Examiner-John P. McIntosh Attorney-James 0. Dixon,Andrew M. Hassell, Harold Levine, Melvin Sharp, Michael A. Sileo, Jr.,Gary C. Honeycutt, John E. Vandigriff, Richard L. Donaldson and StephenS. Sadacca [57] ABSTRACT An improved furnace boat has been developed forthe epitaxial solution growth of Ill-V compounds. The boat comprises oneor more closed compartments wherein the growth solution or solutions arelocated, in combination with a sliding substrate support member at thebase of the solution compartments, such that the substrate wafer can bemoved quickly and easily into contact with, or out of contact with, thegrowth solution. The technique permits optimum surface protection of thesubstrate prior to immersion, removal of the slice at any time incrementof the growth and/or cooling cycle, and will ensure complete removal ofgallium solution from the grown surface without damage to the surface.

6 Claims, 2 Drawing Figures SLIDING FURNACE BOAT APPARATUS Thisinvention relates generally to the epitaxial solu tion growth ofcrystals, and more particularly to the growth of semiconductor crystals,including III-V compound semiconductor crystals, for example. Animproved furnace boat is provided which includes a sliding substratesupport member at the base of one or more solution compartments,including means for complete removal of gallium solution from the grownsurface upon displacing the substrate from the growth solution orsolutions.

Various techniques for the epitaxial solution growth of semiconductorcrystals are known to the industry. For example, the tipping method hasbeen used, wherein a suitable substrate, such as a gallium arsenideslice, is held in one end of a container wherein a suitable growthsolution is kept separate from the substrate simply by tilting thecontainer. When the container and its contents are heated to a suitabletemperature, the entire furnace and container are moved to a levelposition thereby immersing the substrate slice. A cooling program isthen carried out, during which interval super-saturation produces sourcematerial for epitaxial growth. After suitable growth, the furnace isagain tilted to its initial position, which is intended to decant thesolution from the surface of the slice. Actually, however, suchdecanting usually fails to occur, leaving a substantial accumulation ofliquid on the surface of the substrate slice. Upon cooling to roomtemperature, additional, undesired, irregular growth occurs. Moreover,excessive decomposition of the substrate surface frequently occursduring the initial heating step before immersion.

It is also known simply to clip a vertically held substrate into acrucible containing a suitable growth solution at a first temperature,and then to institute a cooling program, followed by removal of thesubstrate by simply lifting the slice out of contact with the solution.Unfortunately, a hard crust commonly forms over the surface of the meltwhich frequently prevents efficient removal of the slice. Otherdetrimental effects include uneven growth, etching and breakage of theslice upon removal from the growth melt.

Accordingly, it is an object of the present invention to provide asystem for the epitaxial solution growth of semiconductor crystals whichensures surface protection of the substrate slice prior to immersion inthe growth solution. A further object is to provide a method whichallows removal of the slice at any time increment of the growth orcooling cycle. A further object of the invention is to ensure completeremoval of gallium solution from the grown surface without damage tothat surface.

Still further, it is an object of the invention to provide an improvedfurnace boat system capable of achieving each of the above objects.

One aspect of the invention is embodied in a furnace boat structurehaving a body member that includes one or more enclosed chambers forholding a suitable melt solution in communication with a slidablesubstrate support member having a recessed area therein for retainingthe semiconductor wafer or other substrate seed. A preferred embodimentincludes means for cleaning the substrate upon sliding the substrate support member to displace the substrate in contact with, or out of contactwith, the growth solution.

The structure is basically a closed box arrangement constructed ofgraphite, for example, or other known furnace boat material such assilicon carbide. The slidable substrate support member is preferablydesigned as a push-pull rod which can be displaced in either directionwithout removing the boat from the furnace. The structure is readilyadapted to move a slice or substrate under a saturated growth solution,or more than one growth solution sequentially, or to move a plurality ofslices in contact with a like plurality of separate growth solutions, orthe reverse of any one of these options. It is also possible to retainthe substrate or substrates in a stationary position and to displace oneor more melts in contact with the substrates either simultaneously or insequence.

Thus, it will be apparent to those skilled in the art that the slidingboat of the present invention can be employed in the growth of a singlelayer on a single substrate, or the growth of a single layer on many substrates, or the growth of sequential layers on a single substrate, orthe growth of sequential layers on a plurality of substrates. It isparticularly advantageous to be able to terminate growth accurately andconclusively at any desired thickness, composition or geometry.

Another aspect of the invention is embodied in a furnace boat structurecomprising a body member having at least one chamber therein for holdinga suitable growth solution, and a slidable substrate support memher atthe base of said chamber having a recessed area therein for retaining asubstrate, and means for adjusting the depth of the recessed area forthe accomodation of different substrate thicknesses. A third embodimentof the invention combines the adjustable recess depth of this embodimentwith the cleaning means of the previously described embodiment.

FIG. 1 is a cutaway perspective view, partially in cross-section, of thesliding boat system of the invention.

FIG. 2 is a fragmentary view, in cross-section, of the boat system ofFIG. 1, wherein the position of the pushrod has been shifted to bringthe substrate in contact with a wiping means, which illustrates a keyfeature of the invention.

In FIG. 1 the rectangular elongated body structure 11 is provided withspaced apart, enclosed compartments or chambers 12 and 13 filled withmolten growth solutions 14 and 15 respectively. Graphite felt members 16and 17 are also provided within body member 11 for the purpose ofcleaning the substrate member as it passes underneath. A slideablepush-rod 18 extends the full length of the boat system and includes atleast one recessed substrate support member 19, the depth of which isadjustable by means of screw-threads 20. Support 19 is adjusted toprovide no more than sufficient space to accomodate substrate wafer 21,such that efficient wiping action is assured whenever the push-rod isdisplaced thereunder.

A fragmentary cross-sectional view of the wiping action is shown in FIG.2. It is particularly significant that the graphite felt or othersuitable wiping means is loaded in compression whereby a constant forceis exerted downward thereby increasing the efficiency of the wipingaction.

For example, the sliding boat system of the invention is operated in thefollowing manner. A substrate of a suitable material, including forexample gallium arsenide, gallium arsenide phosphide, gallium phosphide,

etc., is held in the recessed cylindrical space provided by supportmember 19 so that the wafer surface is within a few mils (for example,0.003 inches) of the upper surface of the push-rod 18, which fits snuglyinto grooves or slots milled into the body of the boat, which togetherwith the tightly fitting walls thereof ensure little or no verticalmovement of the push-rod. As illustrated in FIG. 1, compartments l2 and13 have a conical shape which contains the appropriate solutions, suchas gallium plus gallium arsenide, plus aluminum or tellurium for growthof n-type gallium aluminum arsenide layers. A similar solution isprepared containing zinc instead of tellurium for the growth of p-typelayers.

The tight fit of push-rod 18 effectively seals compartments l2 and 13 toprevent leakage of the liquid melts. The confined space above thesubstrate wafer limits any tendency for the accumulation of gas andthereby resists the amount of arsenic vapor, for example, that canevolve from the surface of a gallium arsenide substrate, therebypreventing decomposition damage to the surface of the substrate in thehigh temperature environment prior to the the growth cycle. Stillfurther, the limited volume restricts the amount of oxygen or otherambient gas which contacts the wafer during the time it is out ofcontact with the growth solutions. Otherwise, an oxide coating wouldform on the substrate thereby preventing optimum growth. Between eachsolution-containing compartment and the central body member are locatedpads of graphite felts 16 and 17 (commercially available material) whosevertical dimension exceeds the heighth of the space provided. Thus, whena lid or other top member is fastened down, compressive force isestablished in the resilient graphite felt. Quartz micro-fiber,fiber-fax and the like are also suitable. The compression of the padsensures efficient cleaning action.

In operation, the assembly is moved to the center of a uniform hot zonewithin a suitable furnace. A vacuum or inert gas purge or otherappropriate means is used to remove air. Commonly, a pure hydrogenatmosphere is used during the high temperature cycle. After the assemblyhas reached equilibrium at the saturation temperature, the slice ismoved from the center of the system to the far end under the n-typesolution. A controlled temperature decrease is carried out, commonlyover a small temperature span, for example, 0.00l to 0.05 per minute for15 minutes to 60 minutes. At the end of this period, the slice is movedback to the center which enables graphite pad 16 to wipe off anyremaining gallium solution. Then the slide is moved on farther toposition the slice under the other solution for the growth of a p-typelayer. A further controlled temperature program is carried during whichtime a p-layer is deposited on the preceding n-layer. Note that graphitefelt pad 17 provides a second cleaning of the wafer surface upondisplacement to its position in communication with melt 15.

At the end of the p-growth, the slide is returned to center again,thereby wiping off the surface under pad 17. The assembly is eithercooled in place to tempera ture, or mechanically moved outside the hotzone for immediate cooling. Note that the newly grown layers are againprotected by the central graphite body which provides spacing that iscloser by the amount of growth. The result is a mirror-smooth, even,planar surface. The applicability of this invention to continuousprocessing will be apparent to those skilled in the art. That is, smallindividual boats carried by a conveyor belt through a temperatureprofiled furnace with semiautomatic mechanical movement of the slidewill provide high volume production.

Multi-layer capability merely requires longer slides and boats.Ultimately, the limitation becomes cost and convenience, instead ofmethod or technique.

It should be apparent that the system of the invention is capable ofproviding epitaxial layers of more accurately controllable thickness,superior flatness, uniformity of surface, and having extremely highcrystal perfection. Additional features include minimum handling ofsubstrates which leads to minimum contamination and minimum cycle time.

What we claim is:

1. A furnace boat structure comprising a body member having at least onechamber therein for holding a growth solution, and a slideable substratesupport member at the base of said chamber, said support member having arecessed area therein for retaining a substrate, said recessed areahaving a screw threaded base for adjusting the depth of said recessedarea for the accommodation of different substrate thicknesses.

2. A furnace boat structure as defined by claim 1 further includingmeans for cleaning the surface of said substrate upon sliding saidsupport member to displace the substrate into contact with, or out ofcontact with said growth solution, said cleaning means comprising afibrous pad mounted in compression adjacent said substrate supportmember.

3. A furnace boat structure comprising a body member having at least onechamber therein for holding a melt, a slideable substrate support memberin communication with a lower portion of said chamber, said supportmember having a recessed area therein for retaining a substrate, and afibrous pad mounted on said body member in compression adjacent saidsubstrate support member for cleaning a substrate in said recessed area,upon sliding said support member to displace the substrate in saidrecessed area in contact with, or out of contact with said melt.

4. A furnace boat structure as defined by claim 3 further includingmeans for adjusting the depth of said recessed area for theaccommodation of different substrate thicknesses.

5. A furnace boat structure comprising a body memher having at least onechamber therein for holding a growth solution, a slideable substratesupport member at the base of said chamber, said support member having arecessed area therein for retaining a substrate,

means for adjusting the depth of said recessed area for theaccommodation of different substrate thicknesses, and a graphite feltpad mounted in compression adjacent said substrate support member forcleaning the surface of said substrate upon sliding said support memberto displace the substrate into contact with, or out of contact with saidgrowth solution.

6. A furnace boat structure comprising a body member having at least onechamber therein for holding a melt, a slideable substrate support memberin communication with a lower portion of said chamber, said supportmember having a recessed area therein for retaining a substrate, and agraphite felt pad loaded in compression for cleaning a substrate in saidrecessed area, upon sliding said support member to displace thesubstrate in contact with or out of contact with said melt.

2. A furnace boat structure as defined by claim 1 further includingmeans for cleaning the surface of said substrate upon sliding saidsupport member to displace the substrate into contact with, or out ofcontact with said growth solution, said cleaning means comprising afibrous pad mounted in compression adjacent said substrate supportmember.
 3. A furnace boat structure comprising a body member having atleast one chamber therein for holding a melt, a slideable substratesupport member in communication with a lower portion of said chamber,said support member having a recessed area therein for retaining asubstrate, and a fibrous pad mounted on said body member in compressionadjacent said substrate support member for cleaning a substrate in saidrecessed area, upon sliding said support member to displace thesubstrate in said recessed area in contact with, or out of contact withsaid melt.
 4. A furnace boat structure as defined by claim 3 furtherincluding means for adjusting the depth of said recessed area for theaccommodation of different substrate thicknesses.
 5. A furnace boatstructure comprising a body member having at least one chamber thereinfor holding a growth solution, a slideable substrate support member atthe base of said chamber, said support member having a recessed areatherein for retaining a substrate, means for adjusting the depth of saidrecessed area for the accommodation of different substrate thicknesses,and a graphite felt pad mounted in compression adjacent said substratesupport member for cleaning the surface of said substrate upon slidingsaid support member to displace the substrate into contact with, or outof contact with said growth solution.
 6. A furnace boat structurecomprising a body member having at least one chamber therein for holdinga melt, a slideable substrate support member in communication with alower portion of said chamber, said support member having a recessedarea therein for retaining a substrate, and a graphite felt pad loadedin compression for cleaning a substrate in said recessed area, uponsliding said support member to displace the substrate in contact with orout of contact with said melt.